These fatty acids -- found most commonly in certain fish -- are known to help reduce inflammation and oxidative stress on cells. Both of those processes can damage nerve tissue, according to the study authors.
Inflammation and oxidative stress have long been linked with ALS, the study authors said, so any nutrient that fights those processes might be helpful.
In the study, "individuals with higher dietary intakes of total omega-3 polyunsaturated fatty acids -- an essential type of dietary fat found in vegetable oils and fish -- had a reduced risk for ALS," said lead researcher Kathryn Fitzgerald of the Harvard School of Public Health in Boston.
"We also found that higher dietary intake of alpha-linolenic acid, a type of fatty acid found in vegetable oils and nuts, is also associated with lower ALS risk," she said.
For the study, Fitzgerald's team looked at the association between ALS and these fatty acids among almost 1,000 ALS patients. They found that those who ate the most foods containing omega-3 fatty acids had the lowest risk of developing ALS.
People ranked in the top 20 percent in terms of their omega-3 fatty acid intake cut their odds of developing ALS by a third, compared to those in the bottom 20 percent, the study found.
Fitzgerald cautioned, however, that the study was an observational study, where the researchers look at data from published sources and not from their own randomized trial. "So we can't say there's a cause-and-effect relationship, only that there's an association," she said.
And there was another caveat: This study only looked at the risk of developing ALS. Whether high intake could help treat people who already have the disease isn't known.
"Future studies are needed to establish whether increasing omega-3 intake might be helpful for people with ALS," Fitzgerald said.
ALS is a relatively rare disease, she noted. "Currently, there are roughly 20,000 to 30,000 Americans who have ALS, and roughly 5,000 patients are diagnosed with ALS each year," Fitzgerald said.
The report was published online July 14 in JAMA Neurology.
Dr. Michael Swash is a British neurologist at the Royal London Hospital and the author of an accompanying journal editorial. He believes that the new study "is important in that it provides the possibility of an environmental factor [diet] in the complex processes triggering the onset of ALS."
Dietary factors could be such a factor, and this research opens the door a little toward addressing that idea, Swash said.
"Maybe we are headed toward two forms of therapy -- one preventing the disorder, an ideal solution -- the other slowing the progression of the disease, also necessary," he said.
For more information on ALS, visit the ALS Association.